Prion diseases are a family of rare progressive neurodegenerative disorders that affect both humans and animals. Such diseases are caused by prions and are distinguished by long incubation periods, characteristic spongiform changes associated with neuronal loss, and a failure to induce inflammatory response.
Prions are the causative agents of transmissible spongiform encephalopathies. A prion is an infectious, abnormal, proteinaceious, transmissible agent that is able to induce abnormal folding of normal cellular prion proteins in the brain, leading to brain damage and the characteristics signs and symptoms of prion disease. Prion diseases are usually rapidly progressive and always fatal.
The prion proteins can occur in both a normal cellular form, which is a harmless protein found in the body's cells, and in an infectious form, which causes disease. The harmless and infectious forms of the prion protein have the same amino acid sequence. However, the infectious form of the protein takes a different conformational form than the normal protein. Sporadic prion diseases may develop when an individual's normal prion proteins spontaneously change into the infectious form of the protein and subsequently alter prion proteins in other cells in a chain reaction. Once they appear, abnormal prion proteins aggregate, or clump together.
About 5 to 10 percent of all Creutzfeldt-Jakob Disease (CJD) cases are inherited. These cases arise from a mutation in the gene that controls formation of the normal prion protein. While prions themselves do not contain genetic information and do not require genes to reproduce themselves, infectious prions can arise if a mutation occurs in the gene for the individual's normal prion protein. Several different mutations in the prion gene have been identified. The particular mutation found in each family affects how frequently the disease appears and what symptoms are most noticeable.
Other risk factors and causes of prion diseases include, but are not limited to, age; exposure to contaminated meat, such as beef; and exposure to contaminated body tissues, such as, human growth hormone, dura mater, corneas, spinal cord, brain tissue, and central nervous system (CNS) fluid.